The present invention relates generally to inverter controls, and more particularly to a control for operating switches in a pulse width modulated inverter to minimize harmonics in the output thereof.
Prior conventional inverters typically include one or more legs, each of which includes a pair of switches connected in series across a DC power source developing first and second voltages. The switches are operated to develop a phase output at a junction therebetween. In such an inverter, each phase output is switched between the first and second voltages developed by the DC power source. This wide swing in output voltage results in an undesirably high harmonic content in the output.
In an attempt to reduce this harmonic content, neutral point clamped inverters have been developed which limit the output voltage swing. Such inverters include a pair of clamping switches coupled between each phase output and a neutral voltage, with the clamping switches being operated by logic in interlocking fashion with the main phase switches in the inverter leg so that the output voltage swing is limited to half the voltage produced by the DC power source. An example of such an inverter system is disclosed in an article entitled "A New Neutral-Point-Clamped PWM Inverter", by Nabae et al., IEEE Transactions on Industry Applications, Vol. 1A-17, No. 5, September/October, 1981, PP 518-523.
The above-referenced article appears to suggest that a memory may be utilized to store a number of pulse width modulated (PWM) switching patterns, with the switches in the inverter being controlled in accordance with a pattern which is selected from the memory based upon an output parameter.
The chief disadvantage of this approach is that output regulation is accomplished in discrete steps, i.e. there is only a discrete number of PWM patterns which are available to control the inverter switches. The occasion may arise where optimal control of the inverter output can be obtained only by use of a PWM pattern not stored in the memory. In such a case, a PWM pattern which is stored in the memory must be used to control the switches, which pattern may not result in adequate control over the inverter output.
This discrete regulation typically means that a large memory must be used to store a large number of PWM patterns to maintain output regulation within acceptable limits under dynamic conditions or that the inverter system be limited to use in applications in which output regulation is not critical.
Further, the above system generates waveforms of differing harmonic content. This variation in harmonic content requires that the output filter with which the inverter is used be sized to accommodate worst case conditions. A consequence of this is that the harmonic content in the output as measured over the total operating range of the inverter is not minimized.